The presence of small non-coding microRNAs (miRNAs) which regulate mRNA stability has become increasingly appreciated as a critical factor in fine-tuning specific neuronal protein levels to mediate diverse brain functions. This widespread influence of miRNA regulation on neuronal physiology suggests that perturbations in miRNA function are involved in the pathogenesis of complex neurodegenerative disorders such as Alzheimer's disease (AD). However, despite initial insights that miRNAs contribute to amyloid pathology during disease progression, the field remains in its infancy and must expand to focus on identifying key multifarious miRNA network changes occurring during the onset of AD which will drive novel therapeutic targets. In particular, whether miRNA networks are dysregulated in the brains of people in the prodromal stages of AD such as amnestic mild cognitive impairment (aMCI) and the extent to which these changes have physiologic consequences for AD progression remain underexplored. To this end, our preliminary microarray and quantitative PCR (qPCR) studies discovered two families of miRNAs, miR-212/132 and miR-23a/b, that were down-regulated in the frontal cortex of aMCI subjects compared to controls. Human miRNA databases revealed that the down-regulation of either miR-212/132 or miR-23a/b was predicted to up-regulate two targets that interact to mediate neuroprotective cell stress responses, the deacetylase sirtuin 1 (sirt1) and the forkhead transcription factor foxo3a;pilot qPCR studies using the same frontal cortex samples revealed that both sirt1 and foxo3a mRNA levels were higher in aMCI compared to controls. Given the relatively delayed involvement of frontal cortex in AD pathogenesis and the ability of this region to respond to the onset of dementia by neuronal reorganization, these data suggest that miRNA-mediated up-regulation of the sirt1/foxo3a pathway represents a compensatory neuroprotective response to mounting disease. In fact, qPCR analysis performed on temporal cortex, an area affected early in the progression of AD, showed no changes in miR-212, miR-23a, sirt1, or foxo3a transcripts in the aMCI subjects. Moreover, pilot in vitro mechanistic studies showed that the coordinated down-regulation of miR-212 and miR-23a increased Sirt1 and Foxo3a protein expression and provided neuroprotection from ?-amyloid toxicity in human neuronal cells. Hence, our preliminary data suggest that we have uncovered a novel miRNA-mediated neuroprotective pathway activated during prodromal AD. This proposal will test this hypothesis using human tissue molecular, biochemical, and histochemical approaches as well as mechanistic pathway modeling in human neurons. These studies may reveal new insights into gene regulation pathways leading to innovative therapeutic avenues for modifying AD progression.

Public Health Relevance

MicroRNA (miRNA) regulation of neuronal gene networks affects a wide variety of complex cellular processes;hence, multifarious miRNA function could influence the progression of neurodegenerative disorders such as Alzheimer's disease (AD). Based on our exciting pilot studies, this proposal will test the extent to which coordinated down-regulation of two related miRNA families activates a neuroprotective pathway during the prodromal stages of AD. Our findings will validate the novel proposition that innate compensatory miRNA-mediated pathways are activated early in AD progression. A greater understanding of these miRNA pathways during the onset of AD will reveal innovative targets for drug discovery, biomarker development, and disease modification.